This invention relates to a circuit arrangement for operating a discharge lamp, comprising
input terminals for connection to a supply voltage source, PA1 means I coupled to the input terminals for generating a low-frequency alternating current from a supply voltage delivered by the supply voltage source, and PA1 means II coupled to the input terminals for generating from the supply voltage a further current which is superimposed on the low-frequency alternating current.
Such a circuit arrangement is known from U.S. Pat. No. 4,187,448. The known circuit arrangement is supplied with a low-frequency AC voltage. The means I are formed by a ballast coil. The means II are formed by a DC-AC converter which generates a high-frequency alternating current which forms the further current. Since a discharge lamp operated by means of the circuit arrangement is provided with current both by the means I and the means II, the dimensions of the ballast coil can be chosen to be comparatively small. In addition, less stringent requirements can be imposed on the DC-AC converter than if this converter were to supply the complete lamp current. As a result, the DC-AC converter can be realised with comparatively inexpensive components. The circuit arrangement as a whole is thus less voluminous than a conventional ballast which comprises exclusively a ballast coil, and also less expensive than a completely electronic ballast which generates a lamp current comprising exclusively a high-frequency alternating current. It is, in addition, possible to render the current supplied by the DC-AC converter adjustable and thus to render the power consumed by the discharge lamp adjustable over a certain range. With the use of such a DC-AC converter in combination with a control loop it is possible to control the total power consumed by the discharge lamp at a substantially constant level independently of, for example, the amplitude of the supply voltage.
A disadvantage of the known circuit arrangement is that the discharge arc exhibits instabilities in some discharge lamps, more in particular in high-pressure discharge lamps, if the lamp current contains a high-frequency component within a lamp-dependent frequency range. These instabilities of the discharge arc render the known circuit arrangement unsuitable for operating such lamps.